A conventional vacuum pump cooling technology is implemented in a structure that a cylinder body is directly cooled by forming a cooling water passage at an outer wall of a pump body cylinder, so a high temperate heat generating during a compress process in which a rotor intakes and exhausts into the interior of a cylinder is directly transferred to a rotor and is heated and thermally expanded, whereas the cylinder body is cooled by means of a cooling water circulating through a cooling water passage formed at an outer wall, so no thermal expansion occurs. So, there are formed a gap between each rotor and the cylinder and a gap between a pair of rotors in view of a thermal expansion which is different between the cylinder body and the rotor, both of which gaps are necessarily made bigger in order to prevent adhesive sticking of a pump. In this case, a high vacuum level cannot be easily obtained due to the bigger gaps or since an exhaust speed is lowered, it takes long time to reach a desired vacuum degree or it becomes impossible to reach a desired vacuum degree. Since it is impossible to control overheating of a rotor, the pump might be adhesively stuck in the course of operation by means of an over thermal expansion of the rotor as compared to the cylinder.
When a high vacuum pump having about 10−3 Torr of vacuum degree is needed in the conventional pump structure, two three-stage pumps are needed. Two pumps are connected without considering connecting more pumps in a multistage pump structure for the reasons that when the pump structure is made in at least three pump structure, an overheating problem due to the increase in the compression ratios and the difference of the thermal expansion of each member cannot be overcome. So, it is possible to a high vacuum degree when a six-stage cylinder structure by connecting two three-stage pumps in series. In this case, connecting two three-stage pumps leads to a long passage of working gas, and the prices of the pumps and maintenance cost significantly increase (about two times maintenance costs are needed when installing two pumps as compared to one pump).